Saguinus labiatus lahiatus and S. juscicollis weddelli form stable mixed-species
groups in the wild. This thesis investigates the costs and benefits of such an association, with
particular emphasis on determining species differences, the "behavioural differences
hypothesis". A nine month field study was conducted in northern Bolivia, which showed that
the species differed in mean height used in the forest, method of locomotion, and preferred
insect foraging strategies.
Subsequent investigations were conducted on captive single species and mixed-species
groups at Belfast Zoo. Preliminary observations of mixed-species groups in standard
enclosures and while free-ranging, demonstrated that they behaved in a similar way to their
wild counterparts, where the S. labiatus had priority of access to food and utilised a higher
mean height in the cage. Therefore, it is reasonable to relate the results of captive
investigations to the wild situation.
The "behavioural differences hypothesis" was investigated through the presentation of
novel objects in various parts of the environment. Both species' reactions to objects varied
according to predictions based on their vertical partitioning. S. labiatus were found to use a
more visually orientated approach than S. juscicollis, and this can be related to insect foraging
strategies in the wild.
An experiment was conducted with novel food and non-food objects, in order to test
the "social facilitation hypothesis", which predicts that the behaviour of one species can
orientate the other towards the presence of food. Results suggest that S. labiatus have priority
of access to objects and are first to consume food, but S.juscicollis are able to use this
behaviour and always gained some food. The results suggest that overall both species benefit
from increased foraging efficiency.
The hypothesis that individuals in mixed-species groups benefit from decreased
predation through increased vigilance was investigated, and supported, as members of both
species were shown to benefit from a decrease in individual vigilance effort, but an overall
increase in total vigilance per unit time. This was shown in both general vigilance and in the
active monitoring of a threatening stimuli.
Findings are discussed in terms of costs and benefits towards the participants in mixedspecies
groups, and are compared to the main theoretical viewpoints in the

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